1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor or other multilayer ceramic electronic device having internal electrode layers formed using a specific composition of conductor paste and a method of production of such a device.
2. Description of the Related Art
A multilayer ceramic capacitor, an example of a multilayer ceramic electronic device, is comprised of a element body structured by dielectric layers and internal electrode layers alternately stacked in a plurality of layers and a pair of external end electrodes formed at the two ends of the element body. This multilayer ceramic capacitor is produced by first alternately stacking pre-firing dielectric layers (ceramic green sheets) and pre-firing internal electrode layers (predetermined patterns of electrode layer conductor paste) in the necessary number of layers to produce a pre-firing element body, then firing this and forming a pair of external end electrodes at the two ends of the pre-firing element body.
When producing a multilayer ceramic capacitor, the pre-firing dielectric layers and pre-firing internal electrode layers are simultaneously fired, so the conductive material included in the pre-firing internal electrode layers is required to have a melting point higher than the sintering temperature of the dielectric material powder included in the pre-firing dielectric layer, to not react with the dielectric material powder, to not diffuse in the fired dielectric layer, etc.
In recent years, to satisfy these requirements, for the conductive material included in the pre-firing internal electrode layers, the conventionally used Pt, Pd, and other precious metals have been replaced by reducing the sintering temperature of the dielectric material powder and using Ag—Pd alloy for the conductive material included in the pre-firing internal electrode layers or by imparting reduction resistance to the dielectric material and using Ni or other inexpensive base metal able to be fired in a reducing atmosphere.
The case of use of Ni for the conductive material included in the pre-firing internal electrode layers will be illustrated. Ni is lower in melting point compared with the dielectric material powder included in the pre-firing dielectric layer. For this reason, when simultaneously firing the pre-firing dielectric layers and the pre-firing internal electrode layers including Ni as the conductive material, due to the difference in sintering start temperature between the dielectric material powder and Ni, along with the progress in sintering of the dielectric material powder, the Ni grains grow and in the end the layers tend to break. As a result, the coverage rate of the internal electrodes tends to fall.
Therefore, to suppress grain growth of Ni due to firing, that is, to impart an Ni sintering suppression effect, and, as a result, improve the coverage rate of the internal electrodes and further improve the bondability between the Ni internal electrodes and dielectric layers, the practice has been to add to the conductor paste for forming the internal electrode layers the dielectric material (for example, barium titanate powder) included in the dielectric layers for co-material.
However, if the amount of addition of the co-material in the conductor paste is too great, the sintering suppression effect and the bondability between the Ni internal electrodes and dielectric layers are improved, but the continuity of the Ni internal electrodes facing each other across the dielectric layers tends to deteriorate. As a result, Ni neck growth occurs, the Ni concentrates in stripe shapes resulting in a sparse conductor structure of the internal electrodes, and the coverage rate of internal electrodes drops making it impossible to obtain a large electrostatic capacity.
On the other hand, in cases where the amount of addition of the co-material in the conductor paste is too small, the coverage rate of the internal electrode does not drop, so a large electrostatic capacity can be obtained, but a sufficient sintering suppression effect and bondability of the Ni internal electrodes and dielectric layers can no longer be obtained and cracks are also caused.
To obtain a larger electrostatic capacity with a small size, both the internal electrodes and dielectric layers have to be made as thin as possible (reduction of thickness) and as many of these as possible stacked (increase of layers), but if just reducing the thickness and increasing the layers of the internal electrodes and dielectric layers, the ratio of the internal electrodes inside the pre-firing element body increases and more of the co-material has to be added to the conductor paste. However, if the amount of the co-material added to the conductor paste becomes greater due to the above reasons, the desired electrostatic capacity can no longer be obtained.
Therefore, in recent years, means for obtaining the desired electrostatic capacity without adding a large amount of co-material have been proposed (see Japanese Patent Publication (A) No. 2000-269073).
Japanese Patent Publication (A) No. 2000-269073 proposes the technology of adding to the conductor paste used for forming the internal electrodes additives having an Ni sintering suppression effect, that is, an oxide of at least La and Cr (La2O3, Cr2O3) so as not to influence the continuity of the internal electrodes and as a result not reduce the electrostatic capacity even if reducing the amount of addition of the co-material. Note that Japanese Patent Publication (A) No. 2000-269073 discloses the average particle size of the additive being 0.5 μm or more. Further, it describes adding the above additive of this size in an amount, with respect to Ni powder as 100 wt %, of 0.5 wt % or more.
However, with the technology of Japanese Patent Publication (A) No. 2000-269073, the additive added to the Ni powder was large in average particle size and the amount of addition was great, so the continuity of the Ni internal electrodes became poorer. As a result, the coverage rate of the internal electrodes dropped and therefore a large electrostatic capacity could no longer be obtained in some cases.